18inch tire gearing on 450r

[quadgod440ex]

Hello everyone , Im going to be running a set of 18 inch tires. I would like the gearing to be the same as it is right now. Which is stock . Im running the stock 400ex tires in the rear but when i switch to the 18's my gearings is gonna change. Can anyone tell me what the ratio would be to make the gearing like stock again ?
Thanks for any of the help!

[kbass24emtp]

Unless you are going to run wide open for a period of time, the 18" tires are only going to effect your top end speed a few mph. You will hit the rev limiter on long wide open runs, this could be because I have the HRC kit also. If your worried about loosing speed, go up one tooth on your counter sprocket to a 15 or drop three theeth from your rear sprocket.